The marvelous xJane sent me a link to this spider pillow on Etsy, along with the comment:

This would be super easy to make oneself: just applique/embroider some spiders onto a damask-style pillow covering (either before you make the pillow case if you’re so inclined, or on one that was bought in a store after-the-fact). I think a more subtle one spider off to the side would be cuter, but to each one’s own (and, of course, the limit needn’t be “spiders on pillows”). Cute for a Victorian/gothic themed couch.

I quite agree. The Etsy listing doesn’t offer a lot of detail on the materials, but the tags on the item suggest that the spiders are felt appliques. A standard square pillow is very easy to make, and if applique isn’t your thing (I’m really not sure how well the felt would hold up over time), you could embroider or even paint the design on the fabric. I like the elegant look of the jacquard fabric, and if you nestled a little spider up in the corner it would be a secret bit of goth on an otherwise normal-looking pillow.

The seller has several other appliqued pillows with darkish themes; I quite like the Nevermore raven one as well.